Image sensors used to photograph subjects and convert the photographed images into electrical signals are used in cameras mounted on a vehicle, a security apparatus, and a robot, as well as in general consumer electronics, such as a digital camera, a mobile phone camera, and a portable camcorder. Such an image sensor may include a pixel array and unit pixels included in the pixel array may respectively include photo detecting devices. The photo detecting devices may generate electrical signals in accordance with the intensities of absorbed light. For example, among the photo detecting devices, photodiodes may absorb the light to generate currents.
An image sensor may include transistors for controlling photo detecting devices, a circuit for driving a pixel array, and a circuit for measuring output signals of the pixel array as well as the photo detecting devices. The photo detecting devices, the transistors for controlling photo detecting devices, the circuit for driving a pixel array, and the circuit for measuring output signals of the pixel array may be respectively formed by semiconductor manufacturing processes. For example, the electrical signals generated by the photo detecting devices may pass through at least one transistor to be converted into output signals whose voltages change in accordance with the intensities of the absorbed light and the pixel array may output the output signals external to another circuit or device.